1. Field of the Invention
The present invention relates to an electrophotographic sensitized body which is particularly suitable for laser beam printers using a semiconductor laser.
2. Description of the Related Art
An electrophotographic sensitized body is provided with a photoconductive layer which comprises photoconductive material on the surface of a metallic substrate. As the photoconductive material with high resistance used for the photoconductive layer of this electrophotographic sensitized body, amorphous semiconductor, e.g. hydrogenated amorphous silicon, is given attention. This material shows high photosensitivity in the visible light range, high hardness and low toxicity, compared with the conventional photoconductive material comprising amorphous selenium or organic photoconductor. However, the photosensitivity around 780-800 nm, the region of oscillatory wavelength of the semiconductor laser, is not high and further sensitization in this region is desired.
To improve the sensitivity in a particular wavelength region, following two conditions are extremely important:
(i) On irradiation of light in the given wavelength region, pairs of electrons and positive holes are readily created in the photoconductive layer. In other words, an optical band gap, corresponding to the wavelength region concerned, must exist in the photoconductive layer.
(ii) The pairs of electrons and positive holes created in (i) must be moved quickly in the photoconductive layer by the electric field, which is produced between positive charges applied on the surface of the sensitized body and negative charges induced on the interface between the substrate and photoconductive layer. (The sign of the charges may sometimes be inverted.) In other words, the mobility of electrons and positive holes in the photoconductive layer must be large.
Particularly in (ii), it is well known that not only the mobility of the electrons which directly neutralize the positive charges on the surface of the sensitized body but that of the positive holes which neutralize the negative charges on the surface of the substrate is important.
In addition to having sufficient sensitivity, the electrophotographic sensitized body must further meet following two conditions:
(iii) The specific resistance of the photoconductive layer must be over 10.sup.10 .OMEGA.cm in order to prevent the discharge of the charges, which have been applied by Corona discharge etc. on the surface of the sensitized body across the thickness of the photoconductive layer before the light exposure.
(iv) After the light exposure, in order to prevent disappearance of the charge pattern formed on the surface of the sensitized body before development due to the charge's lateral mobility, the surface resistance of the sensitized body must be adequately high, i.e. over 10.sup.10 .OMEGA.cm in specific resistance convertedly.
The hydrogenated amorphous silicon usually has an optical band gap of about 1.8 eV, indicating a good photosensitivity for light around 600-650 nm, the region of oscillatory wavelength of the gas laser using He gas or Ne gas, but an abrupt drop in photosensitivity around 780-800 nm (the range corresponding to the optical band gap of about 1.5 eV), the region of oscillatory wavelength of the semiconductor laser. Methods like Ge- and Sn-addition to the amorphous silicon were found to reduce the optical band gap of this material, as is reported, e.g. in "Modern Amorphous Silicon Handbook", pp. 200-201, 221-223 (Mar. 31, 1973) published by Science Forum Co., Ltd. However, these methods lead to an unfavorable result that specific resistance of the sensitized body is reduced.
In order to avoid this drawback, a composition of sensitized body has been proposed, as is detailed, e.g., in Japanese Patent Application Kokai (Laid-Open) No. 219565/83.
Namely, it is the composition in which the hydrogenated amorphous silicon carbide layer, which has a comparatively large optical band gap and specific resistance, is deposited on the photoconductive layer and on the interface between the photoconductive layer and its substrate. This layer on the sensitized body surface is called "surface coating layer", and that on the interface is called "barrier layer". The surface coating layer is effective against lateral redistribution of the charges on the surface and discharge in the direction of the layer thickness. On the other hand, the barrier layer effectively blocks the charge implantation from the substrate into the photoconductive layer. These measures improve photosensitivity in the region of oscillatory wavelength of the semiconductor laser to some extent.
However, investigations by the present inventors have disclosed a problem of contamination in the photoconductive layer by diffusion of the substrate's constituent elements through the barrier layer. The diffusion of the substrate's constituent metal is due to the heating in the processes to prepare the barrier layer, photoconductive layer and surface coating layer. More concretely put, these layers are usually prepared by sputtering, plasma CVD or evaporation process. In these formation processes, the substrate is heated to around 200.degree.-300.degree. C., partial diffusion of the substrate's constituent elements into the barrier layer and photoconductive layer. By this diffusive contamination, an impurity level is formed inside the band gap of the photoconductive layer, or the specific resistance is reduced. For example, when the substrate is made of Al and the photoconductive layer of amorphous silicon, Al contaminates the amorphous silicon reducing the resistance of the sensitized body. Consequently, the effect of electric field on the electrons and positive holes in the photoconductive layer is reduced, the travel efficiency of the electrons and positive holes created by photo-absorption becomes worse and the photosensitivity decreases. Furthermore, the trap level of electrons and positive holes by the diffused metal as impurity in the silicon causes reduction of the mobility.
The phenomenon that the substrate's constituent metals diffuse into the photoconductive layer was observed in all cases where hydrogenated amorphous silicon was used as material for the photoconductive layer, irrespective of the presence of a barrier layer. It was confirmed that the decrease in resistance and the deterioration of photosensitivity of the photoconductive layer were caused by such diffusion of the substrate constituents into the photoconductive layer.